FIG. 1 shows a pixelated OLED device 100, comprising a functional stack on a substrate 102. The functional stack comprises one or more organic functional layers 104 between two conductive functional layers (106 and 108) which serve as electrodes (i.e. anode and cathode). The conductive layers are patterned as desired. For example, the conductive layers can be patterned to form rows of anodes in a first direction and columns of cathodes in a second direction. OLED cells or pixels are located in an active region where the cathodes and anodes overlap. Charge carriers are injected through the cathodes and anodes via bond pads 112 for recombination in the organic layers. The recombination of the charge carriers causes the organic layer of the pixels to emit visible radiation. The device is encapsulated with a cap 110, hermetically sealing the cells.
As shown in FIG. 1, pillars 114 are used to facilitate patterning of the upper conductive layer 108. The pillars are formed on the substrate after the formation of the lower conductive layer 106. Thereafter, the organic layer and conductive layer are deposited. Due to the profile of the pillars, the continuity of the upper conductive layer is disrupted, leaving segments of the conductive layer 108a over the organic layer 104 and segments 108b on top of the pillars.
Referring to FIG. 2(a), the upper conductive layer 108 is deposited selectively onto the active region, through an aperture in a shadow mask 202. The shadow mask is typically in contact with the substrate during deposition. Close contact between the substrate and the shadow mask may lead to the generation of shorts between adjacent electrodes, such as 108a(i) and 108a(ii). Particles 206 accumulated on the shadow mask surface may be transferred to the substrate, causing shorting between electrodes 108a(i) and 108a(ii), as shown in FIG. 2(b). Shorting may also be caused by, for example, damage of the pillar structures by the shadow mask during alignment.
Known methods employed to protect the substrate from damage by the shadow mask include maintaining a gap between the shadow mask and the substrate. This gives rise to a blurring of the edges of the conductive layer and a reduction in precision. The conductive layer is deposited over an area larger than the active area to compensate for the blurring. Hence, the outer dimensions of the OLED device are increased without increasing the useful light emitting active area. Moreover, maintaining the gap between the shadow mask and substrate requires additional control complexity.
As evidenced from the foregoing discussion, it is desirable to provide a method that improves the reliability of electrode patterning in the fabrication of OLED devices.